In some strands of Reformed theology, the regulative principle of worship is understood to mean that "church holidays," not specifically established in the Bible, should not be recognized in public worship. Thus in some denominations, church services around Christmas and Easter make no special mention of the holidays.
John Calvin, of course, was a developer and proponent of the regulative principle of worship, but it's not clear to me how strictly he applied the rule to the case of Christmas. There seems to be an ongoing debate over his beliefs which is a bit difficult to wade through (see, for example, quotes in favor and against).
Here, I'd like to suspend judgment on what he thought, and instead focus on what he did. Did Calvin recognize or observe Christmas during public worship? Did he do so only in certain places or times in his career? And if so, in what way did he observe it? Casual mention in an unrelated sermon, more hymns about Christ's birth than usual, or perhaps a sermon text like Micah 5 or Luke 2?
To be clear, I'm not interested in whether or not Calvin gave gifts on Christmas or otherwise celebrated the holiday outside public worship. I'm only asking about his application of the regulative principle of worship, which applies only to public worship services.